Semiconductor light emitting device

ABSTRACT

At least one recess and/or protruding portion is created on the surface portion of a substrate for scattering or diffracting light generated in a light emitting region. The recess and/or protruding portion has a shape that prevents crystal defects from occurring in semiconductor layers.

RELATED APPLICATIONS

This application is a continuation of application Ser. No. 10/897,163,filed Jul. 23, 2004, which is a divisional of parent application Ser.No. 10/201,600, filed Jul. 24, 2002, which claim priority to JapanesePatent Application Nos. P2001-223114, filed Jul. 24, 2001, P2002-041737filed Feb. 19, 2002, and P2002-213490 filed Jul. 23, 2002, the entirecontents of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a semiconductor light emitting device,in particular, to a nitride-based compound semiconductor light emittingdevice wherein a recess or a protruding portion is provided in asubstrate so that defects do not occur in the semiconductor and thereby,the direction of guided light is changed in a semiconductor layer toincrease the external quantum efficiency.

DESCRIPTION OF THE PRIOR ART

In a semiconductor light emitting device, for example, in a lightemitting diode (LED), an n-type semiconductor layer, a light emittingregion and a p-type semiconductor layer are essentially made to grow ontop of a substrate to form a layered structure while a structure isadopted wherein electrodes are formed on the p-type semiconductor layerand on the n-type semiconductor layer Light, generated throughrecombination of holes and electrons that have been injected through thesemiconductor layers to the light emitting region, is emitted through alight transmitting electrode on the p-type semiconductor layer or fromthe substrate. Here, the light transmitting electrode means an electrodethat allows light to be transmitted through the electrode and that ismade of a metal thin film or of a transparent conductive film formed onalmost the entirety of the p-type semiconductor layer.

In order to control the layered structure of a light emitting diode atatomic level, the substrate is processed so that the flatness thereofbecomes of a level of a mirror surface. Semiconductor layers, a lightemitting region and electrodes on top of a substrate form a layeredstructure wherein the layers are parallel to each other. Since the indexof refraction of the semiconductor layers is high, a light guide isformed between the surface of the p-type semiconductor layer and thesurface of the substrate. That is to say, the light wave guide is madein a structure wherein the semiconductor layers having a high index ofrefraction are sandwiched between the substrate and the lighttransmitting electrode having a low index of refraction.

Accordingly, in the case that light enters the inner-surface of theelectrode or the outer-surface of the substrate at an angle larger thana critical angle, the light is layers trapped within the light guide.The light is reflected at the interface between the electrode and thep-type semiconductor layer and at the surface of the substrate topropagate laterally in the layered structure of the semiconductor. Sincethe light loses its energy during the propagation in the semiconductorlayer, the external quantum efficiency of the device is lowered. That isto say, the light that has entered the interface at an angle larger thanthe critical angle repeat reflection in the light guide and finally beabsorbed. Therefore, the emitted light is attenuated and cannot beeffectively emitted to the outside, which lowers the external quantumefficiency of the device.

A method has been proposed wherein a light emitting diode chip isprocessed to be of a hemispherical form or of a truncated pyramidal formso that light generated in the light emitting region is made to enterthe surface at an angle smaller than the critical angle. However, it isdifficult to make such a chip.

Also, a method has also been proposed wherein the top surface or theside of a light emitting diode is roughened. However, with such amethod, there is a risk that the p-n junction may be partially damagedand the effective light emitting region is reduced.

Another method has been proposed wherein light generated in the lightemitting region is scattered by creating a recess or protrusion in thesurface of a substrate so that the external quantum efficiency isincreased (see Japanese laid-open patent No. 11-274568 (1999)).According to this method, in a GaN-based LED wherein the sapphiresubstrate, n-type GaN, p-type GaN and a transparent electrode aresequentially layered, the surface of the sapphire substrate is randomlyroughed by means of a mechanical polishing or etching. Thereby, lightthat has entered the sapphire substrate is scattered so that theexternal quantum efficiency is increased.

SUMMARY OF THE INVENTION

However, in the above-described conventional light emitting diode, theexternal quantum efficiency may be lowered by the recess or theprotrusion. That is to say, in the case that the surface is roughened atrandom to generate recess or protrusion, the crystallinity of the grownGaN may be lowered. Therefore, the luminous efficiency, i.e. internalquantum efficiency, in the GaN semiconductor layers is lowered, and thusthe external quantum efficiency is lowered rather than raised. Inaddition, if the light absorption within the light guide is so large,the external quantum efficiency does not reach a sufficient level onlywith the randomly roughed surface.

Therefore, an object of the present invention is to provide asemiconductor light emitting device wherein an improved external quantumefficiency can be stably secured.

According to the present invention, a semiconductor light emittingdevice has a light emitting layer and two semiconductor layers which areformed on the surface of the substrate made of different material fromthat of the semiconductor layers. The light emitting region emits lightto outside through the semiconductor layer or substrate. The LED ischaracterized in that at least one recess and/or protrusion is formed onthe surface of the substrate so that the light generated in thelight-emitting region is scattered or diffracted, and that the recessand/or protrusion prevents crystal defects from occurring in thesemiconductor layers. Here, “prevent crystal defect from occurring”means that the recess or protrusion causes neither an morphologicalproblem, such as “pits”, nor increase of dislocations in thesemiconductor layers.

One of the characteristics of the present invention is in that therecesses and/or protrusions, having such shapes as to prevent defectsfrom growing iii a semiconductor layer on the substrate, are formed onthe surface of the substrate. The recesses and/or protrusions are formednot at the interface between the semiconductor layer and the electrode,but at the interface between the semiconductor layer and the substrate.This improves the crystallinity of the light emitting region (activelayer) and increase the output power of the device. In particular, inthe case of a gallium nitride-based component semiconductor lightemitting device, a substrate, an n-side nitride semiconductor layer, alight emitting region (active layer) and a p-side nitride semiconductorlayer are layered, in this order, wherein the film thickness of thep-side nitride semiconductor layer is less than that of the n-sidenitride semiconductor layer. Therefore, recesses or protruding portionsare provided at the interface between the semiconductor layer and thesubstrate rather than at the interface between the semiconductor layerand the electrode and thereby, the effect due to unevenness is mitigatedby the thick n-side nitride semiconductor layer so that thecrystallinity of the light emitting region (active layer) can bemaintained in an good condition.

In the case of a semiconductor light emitting device having aconventional flat substrate, light propagated through the semiconductorlayer in the lateral direction attenuates before emerging from thesemiconductor layer because a portion thereof is absorbed by thesemiconductor layer or by the electrode during propagation. On thecontrary, according to the present invention, light propagated in thelateral direction in the ease of a conventional flat substrate isscattered or diffracted by recesses and/or protruding portions andfinally efficiency emitted from the upper semiconductor layer or thelower substrate. As a result, the external quantum efficiency can begreatly increased. That is to say, first, light flux directed upward ordownward from the substrate increases through the scattering anddiffracting effects of light due to the unevenness so that the frontalbrightness, which is the brightness of the light observed from the frontof the light emitting surface of the device, can be enhanced. Second,light propagated in the lateral direction is reduced through thescattering and diffracting effects of the unevenness so that the totalamount of light emission can be enhanced by reducing the absorption lossduring propagation.

In addition, crystal defects do not increase in the semiconductor layereven in the case that recesses and/or protruding portions are created inthe surface portion of the substrate. Therefore, the above-describedhigh external quantum efficiency can be stably secured. In the presentinvention, it is preferable for the inside of the recesses or thesurroundings of the protruding portions to be completely filled in witha semiconductor layer. This is because, in the case that a cavity existsinside a recess or in the surroundings of a protruding portion, thescattering or diffracting effects are prevented. This lowers theefficiency of the light emission.

Either recesses or protruding portions may be created in the surfaceportion of the substrate. Combination of recesses and protrudingportions may be created. Such combination may provide similar workingeffects. However, protrusions are more preferable than recesses, becauseit is easier to completely fill the surrounding of protrusions ratherthan recesses. If a cavity is remained around the protrusions orrecesses, the scattering or diffracting effects are prevented, whichlowers the output power of the device.

Shapes of recesses and/or protruding portions for preventing the growthof defects in the semiconductor layer are, concretely, shapes having, ascomponent sides, lines that cross a plane approximately parallel to thestably growing face of the semiconductor In other words, if the shapesare observed from the upper side of the substrate, the shapes have lineswhich are unparallel to the stably growing face of the semiconductor.Here, the stably growing face indicates the surface on which the growthrate of the material made to grow is slower than any other surface.Generally, the stably growing surface is observed as a facet during thecrystal is grown. For example, in the case of gallium nitridesemiconductors, the stable growing faces are the ones parallel to the Aaxis (especially, M face). Therefore, the recesses or protrudingportions are formed, when observed from the upper side, in polygon ofwhich component lines are unparallel to the A axis-parallel plane. Inother words, in polygon of which component lines are unparallel to Aaxis. In the case that the recesses and/or protruding portions have, ascomponent sides, lines approximately parallel to the stably growing faceof the semiconductor, crystal defects occur in such portions at the timeof the film growth of the semiconductor layer and these defects lowerthe internal quantum efficiency which causes the lowering of theexternal quantum efficiency.

More concretely, the recesses and/or protruding portions can be, forexample, polygons, triangles, parallelograms or hexagons, and arepreferably equilateral triangles, rhomboids or regular hexagons having avertex in a plane approximately parallel to the stably growing face ofthe semiconductor and having, as component sides, lines that cross theplane approximately parallel to the stably growing face of thesemiconductor.

Here, in the present specification, the phrase “a recess or a protrudingportion is in the form of a polygon” means that the shape of the recessor of the protruding portion in the plan view observed from above is inthe form of a polygon. It is not necessary to form a complete polygon.The edge of the polygons may be rounded as a result of processing.

For example, in the case that a GaN-based semiconductor is made to growon a C plane of a sapphire substrate, the growth starts in hexagonalislands having planes parallel to A axis, which planes are the stablygrowing face of a GaN-based semiconductor, as a component side, andthen, these islands are connected to become a uniform semiconductorlayer. Therefore, a regular hexagon having an A axis as a componentside, is assumed and a recess or a protruding portion is created in apolygon (for example, a triangle, a hexagon, or the like) having, as acomponent side, a line perpendicular to a segment that connects thecenter of the above hexagon and the vertex. A GaN-based semiconductorthat is flat and has an excellent crystallinity can be made to grow ontap of a sapphire substrate wherein unevenness is created in the abovemanner.

In addition, though one recess and/or protruding portion may besufficient for the invention, when a pattern is formed by repeating theshape of a recess or of a protruding portion, the efficiency ofscattering or diffraction of light increases so that the externalquantum efficiency can be further increased. Here, in the presentinvention, even in the case that recesses and/or protruding portions areprovided on a substrate in a repeating pattern, the semiconductor layeris made to grow so that local crystal defects due to recesses or toprotruding portions can be prevented and thereby, the entire surface ofthe substrate can be used as a light emitting surface.

The present invention is characterized in that recesses and/orprotruding portions are created in the surface portion of a substrate toscatter or diffract light. The material itself for the substrate and forthe semiconductor of the light emitting device is not directly relatedto the invention and any material, for example, III-V groupelements-based semiconductors, concretely, a GaN-based semiconductor,can be utilized for a semiconductor layer of a semiconductor lightemitting device. The stably growing face of a GaN-based semiconductorlayer is an M plane {1-100} of a hexagonal crystal. Here, {1-100}represents all of (1-100), (01-10) and (−1010). An M face is one of thefaces parallel to A axis. In some growing conditions, the stably growingfaces of GaN-based semiconductors are the faces parallel to A axis otherthan M faces.

As for the substrate, a sapphire substrate, an SiC substrate or a spinelsubstrate can be used. For example, a sapphire substrate having a Cplane (0001) as a main surface can be used as the above-describedsubstrate. In this case, an M plane, which is the stably growing face ofa GaN-based semiconductor layer, is a plane parallel to an A plane{11-20 of a sapphire substrate. Here, {11-20} represents all of (11-20),(1-210) and (−2110).

The depths of recesses or the steps of protruding portions are 50 Å ormore, and it is important for them to be equal to or less than thedimension of the thickness of the semiconductor layer made to grow onthe substrate. The depths or the steps must be at least λ/4 or more whenthe wavelength of the emitted light (for example, 206 nm to 632 nm inthe case of an AlGaInN-based light emitting layer) is λ in order tosufficiently scatter or diffract light. However, the depths of therecesses or the steps of protruding portions becomes larger than thethickness of the semiconductor layer, which is made to grow on thesubstrate, it becomes difficult for a current to flow in the lateraldirection within the layered structure so that the efficiency of thelight emission is lowered. The surface of the semiconductor layer mayhave recesses and/or protruding portions. Though it is preferable forthe depths or the steps to be of λ/4 or more in order to sufficientlyscatter or diffract light, depths or steps of λ/4n (n is the index ofthe refraction of the semiconductor layer) or more can gain the effectsof scattering or diffraction.

It is important for the size of the recesses and/or protruding portions(that is to say, the length of one side that becomes a component side ofa recess and/or protruding portion) and for the intervals between therecesses anchor protruding portions to be at least the size of λ/4 ormore when the wavelength in the semiconductor is λ (380 nm-460 nm). Thisis because, unless the size is at least λ/4 or more, light cannot besufficiently scattered or diffracted. Though it is preferable for thesize of, and the intervals between, the recesses and/or protrudingportions to be of λ/4 or more in order to sufficiently scatter ordiffract light, size or intervals of λ/4n (n is the index of therefraction of the semiconductor layer) or greater, can gain the effectsof scattering or diffraction. The size of, and the intervals between,the recesses and/or protruding portions may be 100 μm or less from thepoint of view of manufacturing. Furthermore, it is preferable for thesize of, and the intervals between, the recesses and/or protrudingportions to be recesses 20 μm or less in order to increase thescattering surfaces.

Since the total film thickness of the semiconductor layers is, ingeneral, 30 μm or less, it is preferable for the pitch of the unevennessto be 50 μm or less from the point of view of effective reduction in thenumber of total reflection due to scattering or diffraction.Furthermore, it is preferable for the pitch of the unevenness to be 20μm or less from the point of view of the crystallinity of GaN layer.More preferably, the pitch of the unevenness are less than 10 μm. Thisincreases a scattering efficiency and an out-put power of a device.Here, the pitch of the unevenness indicates the minimum distance fromamong the distances between the centers of the neighboring recesses orof the neighboring protruding portions.

Next, as for the shape of the unevenness in the cross section, it ispreferable for a protruding portion to be a trapezoid and for a recessto be a reverse trapezoid, as shown in FIG. 9. Such a shape in the crosssection enhances the efficiency of scattering and diffraction of light.It is not necessary to make the shape in the cross section completelytrapezoidal or reverse trapezoidal. The edge of the trapezoid may berounded during forming the unevenness. Here, a taper angle θ indicates,in the case of protrusions, the angle between the top and side surface,and, in the case of recesses, the angle between the bottom and sidesurface, as shown in FIG. 9. For example, if the angle θ is 90 degrees,the protrusions or recesses has a square cross section. If the angle θis 180 degrees, the protrusions or recesses are flattened. In order tofill the unevenness by the semiconductor, the taper angle θ should belarger than 90 degrees. From the view point of increasing the outputpower by the scattering or diffraction, the taper angle θ is preferablymore than 90 degrees, more preferably more than 105 degrees, much morepreferably more than 115 degrees. On the other hand, too large taperangle decreases a scattering efficiency and induces pits insemiconductor layers. The taper angle is preferably not more than 160degrees, more preferably not more than 150 degrees, much more preferablynot more than 140 degrees.

Here, in the case that the sides of recesses and/or protruding portionsare inclined, the sides and the intervals of the unevenness is definedby the length in the top surface of the substrate (upper surface ofprotruding portions in the case of protruding portions and flat surfaceof the substrate in the case of recesses).

In the present invention, it is preferable to form a metal layer withopenings as an ohmic electrode. In the case an electrode entirelycovering the surface of the semiconductor layer and having openings isformed on semiconductor layers, the electrode could cooperate with theunevenness on the substrate to remarkably increases the utilizationefficiency of the light. Especially, it is preferable that each openingsinclude at least one step portion of the unevenness on the substrate.The reason of this is assumed as follows: First, when the light emittingdevice having the unevenness on its substrate is observed from thefront, step portions of the protrusions and/or recesses seems brighterthan flat portions of the substrate. Accordingly, if openings are formedabove the step portions of the protrusions and/or recesses, the outputpower of the device is remarkably improved. Second, in a device havingthe unevenness on the substrate, light that inherently propagateslaterally or downwardly is scattered or diffracted to go upwardly.However, if a conventional transparent electrode is formed to cover theentire surface of the device, the scattered or diffracted light ispartly absorbed and weakened by the electrode. Accordingly, on asemiconductor layer on a substrate with the unevenness, an electrode,which may be either transparent or opaque, with openings are preferablyformed to expose a part of the semiconductor layer. This helps thescattered or diffracted light to go out of the device and improves theefficiency of the light utilization.

In the case of the gallium nitride semiconductor, including asemiconductor having at least gallium and nitrogen, a portion near theperipheral of the p-side electrode, which is formed on the p-typesemiconductor layer, lights brighter than other portions. By formingopenings in the electrode, not only the light absorption is decreased,but also the length of the peripheral of the p-side electrode, where thelight strongly emits, is increased. Therefore, the efficiency of thelight utilization is improved. It is preferable for L/S≧0.024 μ/μm² tobe fulfilled wherein the total area of the ohmic electrode including theopenings is S and the total sum of the length of the inner periphery ofthe openings is L. This improves the efficiency of the lightutilization, by increasing the length of the peripheral of theelectrode.

As for a material favorable for the ohmic electrode with openings, analloy or a multilayer film including at least one type selected from thegroup consisting of Ni, Pd, Co, Fe, Ti, Cu, Rh, Au, Ru, W, Zr, Mo, Ta,Pt, Ag and oxides of these as well as nitrides of these can be cited.Especially an alloy or a multiplayer film including one type selectedfrom Rhodium (Rh), Iridium (Ir), Silver (Ag) and Aluminum (Al) ispreferable.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view showing a semiconductor light emittingdevice according to a preferred embodiment of the present invention;

FIG. 2 is a view showing an example of a pattern of a recess accordingto the above-described embodiment;

FIG. 3 is a schematic view showing relationships between a stablygrowing face of a nitride semiconductor and a shape of a recess;

FIG. 4 represents views showing manufacturing steps of the firstembodiment;

FIG. 5 represents SEM photographs for observing processes of the growthof gallium nitride on a sapphire substrate wherein protruding portionsare created;

FIG. 6 represents diagrams showing processes of the growth of galliumnitride on a sapphire substrate wherein a protruding portion is created;

FIG. 7 represents diagrams schematically showing manners of propagationof light according to the present invention in comparison with those inconventional structures;

FIG. 8 represents cross sectional views additionally showing otherembodiments;

FIG. 9 is a cross-sectional view of the recess and/or protrudingportions.

FIG. 10 is a graph showing the relationships between the angle ofinclination of a side of a recess and the output of emitted light;

FIG. 11 represents examples of other patterns of a recess or of aprotruding portion;

FIG. 12 represents diagrams for describing other embodiments wherein arecess or a protruding portion is a regular hexagon;

FIG. 13 is graph showing the relationships between L/S (ratio of innercircumference L of an opening to area S of p-side ohmic electrode) andthe output of emitted light;

FIG. 14 represents diagrams showing various variations of the mode ofp-side ohmic electrode;

FIG. 15 represents schematic diagrams showing the relationships betweenthe forms of cross sections of edge portions of the p-side ohmicelectrodes and light emissions;

FIG. 16 is a view of a semiconductor light emitting device, viewed fromabove, according to another embodiment of the present invention; and

FIG. 17 is a view of a semiconductor light emitting device, viewed fromabove, according to still another embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following the present invention is described in detail based onthe concrete examples shown in the drawings. FIGS. 1 and 2 show asemiconductor light emitting device according to a preferred embodimentof the present invention. In these figures a C plane (0001) sapphiresubstrate having an orientation flat in the A plane (11-20) is used as asubstrate 10 while recesses 20 are created in a repeated pattern in thesurface portion of this sapphire substrate 10 In FIG. 2 the substrate isetched so that the hatched portion remains.

This recess 20 forms an equilateral triangle having a vertex in a planeparallel to the stably growing face (1-100), (01-10), (−1010) of theGaN-based semiconductor 11, which grows on the sapphire substrate 10,that is to say, the M plane and having, as a component side, a line thatcrosses a plane approximately parallel to the above-described stablygrowing face. That is to say, from a top view of the substrate, anequilateral triangle that forms a recess 20 has a vertex at a positionwherein the M plane cross each other and each component side of theequilateral triangle crosses the M plane at an angle of 30 degrees or 90degrees. More concretely, as shown in FIG. 3, each component side of arecess 20 is perpendicular to a line segment connecting the center of aregular hexagon having the M plane of GaN semiconductor 11 as acomponent side and the vertex when recess 20 is viewed from above. Whenobserved from directly above the substrate, M faces of the GaN-basedsemiconductor are parallel to A axis.

In addition, the depth of recess 20 is approximately 1 μm and, as forthe size thereof, one side “a” is 10 μm while, as for the intervalsbetween recess 20 and recess 20, one side corresponds to an interval is10 μm.

An n-type GaN-based semiconductor layer 11, an MQW light emitting region12 on n-type GaN-based semiconductor layer 11 and furthermore, a p-typeAlGaN/p-type GaN-based semiconductor 13 on MQW light emitting region 12are formed on top of the above-described sapphire substrate 10.

In the case that a semiconductor light emitting device according to thisexample is manufactured, an SiO₂ film 30 that becomes an etching mask isformed sapphire substrate 10, as shown in FIG. 4A.

Next, a photomask in the shape of an equilateral triangle having a sideof 10 μm is utilized and the photomask is adjusted so that one side ofthe equilateral triangle becomes perpendicular to the orientation flat,wherein each side of the equilateral triangle becomes approximatelyparallel to the plane (1-100), (01-10), (−1010), that is to say, the Mplane, of the sapphire so that SiO₂ film 30 and sapphire substrate 10are etched by approximately 1 μm by means of RIE, as shown in FIGS. 4Band 4C, and after that, SiO₂ film 30 is removed, as shown in FIG. 4D, sothat a repeated pattern of recesses 20, as shown in FIG. 2, is formed inthe surface portion of sapphire substrate 10.

An n-type GaN semiconductor layer 11, an MQW light emitting region 12on-type GaN semiconductor layer 11 and a p-type AlGaN/p-type GaNsemiconductor layer 13 on MQW light emitting region 12 are made to growon top of sapphire substrate 10 having the repeated pattern of recesses20.

Since the lattice of GaN grows with a shift of 30 degrees from a latticeof sapphire substrate 10, the repeated pattern of recesses 20 formed onsapphire substrate 10 forms a polygon having sides approximatelyparallel to the A plane of GaN (11-20), (1-210), (−2110), having avertex in the stably growing face of GaN (1-100), (01-10), (−1010) andnot having a line parallel to the stably growing face of GaN (1-100),(01-10), (−1010), that is to say; the M plane.

These arrangements improves the crystallinity of GaN. The mechanism ofimproving crystallinity will now be discussed with an example ofprotruding portions, since the mechanism is the same as in the case ofrecesses. FIGS. 5A and 5B are SEM photographs of GaN during the processof growth on top of sapphire substrate 10 wherein protruding portions 20in an equilateral triangle shape are created wherein FIG. 5A shows aview as observed from above while FIG. 5B shows a diagonal view fromabove. As shown in FIGS. 5A and 5B, when GaN is made to grow on thesapphire substrate 10, the growth of GaN progresses from the top surfaceof protruding portions 20 and from the flat surface wherein protrudingportions 20 are not created so that the side surfaces and the vicinitythereof of protruding portions 20 are finally filled in with GaN.Accordingly, in the case that the stably growing face of GaN and thesides of protruding portions 20 are parallel to each other, it becomesdifficult for the sides and vicinity of protruding portions 20 to becomefilled in with GaN so that the crystallinity of GaN is lowered.

Therefore, it is preferable to form component sides of protrudingportions 20 to cross (not to become parallel to) the M plane, which isthe stably growing face of GaN. Furthermore, it is preferable, as shownin FIGS. 5A and 5B, for the component sides of protruding portions 20 tobe formed so as to be perpendicular to the line segment connecting thecenter of a hexagon having the M plane, which is the stably growing faceof GaN, as component sides and the vertex. By creating protrudingportions 20 in such a manner, GaN having an excellent crystallinity thatfills in the inside of protruding portions 20 to provide flatness can begained.

It is assumed that this is because the growth rate of GaN becomes higherin a portion wherein GaN that has grown from the top surfaces ofprotruding portions 20 and GaN that has grown from the flat surfacewherein protruding portions 20 are not created make a junction. As shownin FIG. 5B, GaN has grown from the top surfaces of protruding portions20 in the shape of a hexagon having the M plane as component sides. Thegrowth rate of GaN becomes higher in the vicinity of the side planes ofprotrusions, where GaN that has grown from the top surfaces ofprotrusions 20 and GaN that has grown from the flat surface makecontact. Accordingly, the growth of GaN in the vicinity of the sides ofprotruding portions catches up with that in the other regions andthereby, flat GaN is gained.

This is schematically described using FIGS. GA to 6F When, as shown inFIG. 6A, protruding portions 20 are created in sapphire substrate 10 andGaN is made to grow on top of that, GaN grows, as shown in FIGS. 6B and6C, from the top surfaces of protruding portions 20 and from the flatsurface in which protruding portions 20 are not created, while growthslows in the vicinity of the sides of protruding portions 20. As shownin FIGS. 6D and 6E, however, when GaN 11 a, which has grown from the topsurfaces of protruding portions 20, and GaN 11 b, which has grown fromthe flat surface, meet, the growth rate of GaN becomes higher there.Therefore, the growth significantly progresses in the vicinity of sidesof protruding portions 20, wherein growth had been behind. Then, asshown in FIG. 6F, GaN 11 having flatness and an excellent crystallinitygrows. On the contrary, in the case that the surface on which GaN stablegrows and the sides of protruding portions 20 are parallel to eachother, the growth rate does not increase in the vicinity of the sides ofthe protruding portions 20 and therefore, it becomes difficult to fillin the vicinity of the sides of recesses 20 so that the crystallinity ofthe GaN is lowered.

After that, a device process is carried out and electrodes and the likeare appropriately formed so that LED chips completed.

When holes and electrons are injected from n-type GaN semiconductorlayer and p-type AlGaN/p-type GaN semiconductor layer 13 to MQW lightemitting region 12 so that recombination is carried out, light isgenerated. This light is emitted from sapphire substrate 10 or p-typeAlGaN/p-type GaN semiconductor layer 13.

In the case of a semiconductor light emitting device having aconventional flat substrate, as shown in FIG. 7A, when light from lightemitting region 12 enters the interface between p-type semiconductorlayer 13 and the electrode or the surface of substrate 10 at an anglelarger than the critical angle, light is trapped within the light guideso as to propagate in the lateral direction.

On the contrary, in a semiconductor light emitting device of the presentexample, light entering the interface between p-type semiconductor layer13 and the electrode or the surface of substrate 10 at an angle largerthan the critical angle is scattered or diffracted by recess 21, asshown in FIG. 7B, to enter the interface between p-type semiconductorlayer 13 and the electrode or the surface of substrate 10 at an angleless than the critical angle to be emitted.

In the case that the contact electrode on p-type semiconductor layer 13is a light transmitting electrode, the present example is effective foran FU (face up) semiconductor light emitting device and, in the casethat the contact electrode is a reflecting electrode, the presentexample is effective for an FD (face down) semiconductor light emittingdevice. However, if a reflecting electrode has apertures, the presentexample may be used with an FU type. This embodiment is especiallyeffective.

FIG. 8 shows a semiconductor light emitting device according to anotherembodiment of the present invention. The device is formed so that thesides of the steps of recesses 20 are inclined in the embodiment shownin FIG. 8A. In addition, protruding portions 21, in place of recesses20, are formed on the surface portion of substrate 10 in the embodimentshown in FIG. 8B and, in this example, protruding portions 21, of whichthe cross sections are of a semi-circular shape, are formed.Furthermore, an n-type semiconductor layer 11, a light emitting region12 and a p-type semiconductor layer 13 form planes with recesses inaccordance with recesses 20 in the embodiment shown in FIG. 8C.

FIGS. 7C and 7D show examples of light propagation in the embodimentsshown in FIGS. 8A and 8C. It can be seen that light is efficientlyemitted in both cases. In particular, surfaces (sides of recesses or ofprotruding portions) connected to the surfaces of protruding portionsand to the surfaces of recesses having lines (also referred to as thecomponent sides of a polygon), which cross a plane approximatelyparallel to the stably growing face of the semiconductor layers asinterfaces, are formed so as to be inclined relative to the direction inwhich the semiconductor is layered, as shown in FIG. 8A and thereby, theeffects of light scattering or light diffraction notably increase sothat the efficiency of light emission significantly increases. It isconsidered that one factor contributing to this is an increase in thenumber of occurrences of light scattering or light diffraction due toincrease in the area of the surfaces (sides of recesses or of protrudingportions) connected to the surfaces of recesses and to the surfaces ofprotruding portions as a result of the provision of the inclination.

In other words, it is preferable for the shape of the unevenness in thecross section to be a trapezoid in the case of a protruding portion andto be a reversed trapezoid in the case of a recess, as shown in FIG. 9.By providing such a shape in the cross section, the probability ofoccurrence of scattering and diffraction of propagated light isincreased so that the absorption loss of light at the time ofpropagation can be reduced. Here, the taper angle of the sides ofrecesses and/or protrusions indicates, as shown in FIG. 9, the angleformed between the top surface and a side in the case of a protrudingportion and angle formed between the bottom surface and a side in thecase of a recess. For example, if the taper angle is 90 degrees, thecross section of the protrusions and/or recesses will be a square, andif the angle is 180 degrees, the protrusions and/or recesses will becomeflat.

In order to fill the unevenness by semiconductor layers, the taper angleof the protrusions and/or recesses must not be less than 90 degrees.From the view point of improving an output power by an unevenness, thetaper angle of the sides of recesses and/or protruding portions ispreferably more than 90 degrees, more preferably more than 105 degrees,much more preferably more than 115 degrees. On the other hand, too largetaper angle decreases a scattering efficiency and induces pits insemiconductor layers. The taper angle is preferably not more than 160degrees, more preferably not more than 150 degrees, much more preferablynot more than 140 degrees.

FIG. 10 is a graph showing the relationships between the angle ofinclination of the sides of recesses and the LED power. Here, a similartendency as in the graph can be gained when the angle of inclination isregarded as that of the sides of the protruding portions. Thelongitudinal axis of the graph of FIG. 10 indicates the ratio of outputin the case that the LED output when a flat substrate (=taper angle is180 degrees) is used is set as 1 while the lateral axis of the graphindicates the angle of inclination of the sides of recesses. As shown inthe graph, the output of the LED changes significantly when the angle ofinclination (angle formed between the bottom surface of a recess and aside) is changed between 90 degrees and 180 degrees.

FIG. 11 shows examples of other shapes of recesses 20 or protrudingportions 21. In the figure, the hatched portions are the portions thatare not etched.

In addition, in the case that recesses 20 or protruding portions 21 areregular hexagons, the regular hexagons are placed in the direction shownin FIG. 12B, not in the direction shown in FIG. 12C, relative toorientation flat surface A of sapphire substrate 10 shown in FIG. 12A.As described above, in the case that GaN is made to grow on the C faceof the sapphire substrate, the A face of the sapphire substrate and theM face of GaN become parallel to each other, when observed from abovethe substrate. Accordingly, the regular hexagons having uneven surfacesare arranged as shown in FIG. 12B and thereby, each of the componentsides of the regular hexagons becomes perpendicular to any of surface M,which is the stably growing face of GaN. In other word, the hexagonalprotrusions and/or recesses have the component sides that areperpendicular to a segment that connects the center and vertex of thehexagon having the M face of GaN as its component side.

In addition, according to the present invention, a conventionalsemiconductor layer, such as a nitride semiconductor layer, is formed ona substrate in which unevenness is provided so that defects do not occurin the semiconductor and additionally, electrodes and the like axeformed in a device, wherein, though other parts of the configuration arenot specifically limited, remarkable effects are additionally gained bymaking the other parts of the configuration be as follows.

(1) Form and Material of Electrode <1> Open Electrode

It is necessary to provide an electrode on top of the semiconductorlayer on the surface of a semiconductor light emitting device andgenerally, a transparent electrode is formed on the entirety of thesurface of the semiconductor layer when the semiconductor layer is asemiconductor layer having a comparatively high specific resistancewherein current dispersion hardly occurs, such as in a p-type nitridesemiconductor layer. However, at the time when light propagates withinthe light guide formed in the structure of a light emittingelectrode-semiconductor layer-substrate, emitted light is absorbed orattenuated by not only the semiconductor layer but, also, by the lighttransmitting electrode and by the substrate as a result of the effectsof “leakage” of reflected light. In particular, a transparent electrodesignificantly affects the attenuation of emitted light because thegeneral component materials thereof (Au/Ni, for example) has a highratio of light absorption in the short wavelength range.

Therefore, it is preferable to form, as an electrode, a metal filmhaving an opening in a light emitting device according to the presentinvention. Especially, it is preferable that each opening has in itsinside at least one step portion of the unevenness of the substrate. Byforming an electrode with openings on the semiconductor layers, theopenings let the light go through so that the absorption by theelectrode is reduced. It is preferable that a plurality of openings isformed in the metal layer. From the view point of improving anefficiency of light utilization, it is also preferable to make the areaof the openings as large as possible. On the electrode with openings, apad electrode for connecting the device with an outer circuit ispreferably formed.

In addition, in the case of a nitride semiconductor light emittingdevice, in particular, in the case of a gallium nitride-based (at leastgallium and nitrogen are included) semiconductor light emitting device,an electrode having light transmission, preferably through the entiretyof the surface, is, in many cases, provided as a p electrode on thep-type nitride semiconductor layer and then, the device exhibits theproperty wherein the light absorption in the light emitting electrodebecomes great so that the periphery and the vicinity of the periphery ofthe p electrode provided on the p-type nitride semiconductor layer emitslight that is more intense than that emitted from other parts of thedevice. Therefore, openings may be provided in the light transmittingelectrode. Thereby, light absorption is reduced and the peripheralportion that emits intense light is increased in area so that theefficiency of light emission is increased. In this case, it ispreferable for the area of the openings to be provided as large aspossible from the point of view of increase in the efficiency of thelight emission and by making the length of the peripheral portion of thep electrode as long as possible, the efficiency of the light emission isfurther increased.

It is preferable for the electrode formed on the surface of thesemiconductor layer to be an electrode having an opening, as describedabove, because the effect of recesses and/or protruding portions on thesurface of substrate is much higher with an electrode having an opening.There may exist two reasons. First, when observed from the front of thedevice, the brightness of edges of recesses and/or protruding portionsis higher than other portions. Therefore, by forming the openings abovethe edges of the recesses and/or protrusions, the output power isconsiderably increased. Second, light that has reached to upper areasthrough scattering or diffraction has a low intensity. Therefore, mostof the light that has reached to upper areas through scattering ordiffraction is absorbed by the light transmitting electrode in theconfiguration wherein a conventional light transmitting electrode isprovided on the entirety of the surface. In the case that asemiconductor layer is formed on a substrate wherein unevenness isprovided, openings are provided in the light transmitting electrode or anon-light transmitting electrode so that the semiconductor layer ispartially exposed and thereby, light having a low intensity is easilyemitted to the outside so as to significantly increase the efficiency oflight emission.

<2> Material for Open Electrode

As described above, in the case of a nitride semiconductor lightemitting device, in particular, in the case of a gallium nitride-based(at least gallium and nitrogen are included) semiconductor lightemitting device, an electrode having light transmission almost theentirety of the surface of a p-type nitride semiconductor layer isprovided as a p electrode and in a more favorable embodiment, anelectrode provided with openings is formed on almost the entirety of thep-type nitride semiconductor layer so that the efficiency of the lightemission is increased. At this time, a metal or an alloy made of twotypes of metal is used as a material used in the electrode and a singlelayer or a plurality of layers can be formed. A metal material of a highreflectance for at least the wavelength of the emitted light ispreferably used as the material for this electrode. This reduces thecomponents of light absorbed by the electrode so that the efficiency ofthe light emission to the outside can be increased.

As for a material favorable for the open electrode, an alloy or amultilayer film including at least one type selected from the groupconsisting of Ni, Pd, Co, Fe, Ti, Cu, Rh, Au, Ru, W, Zr, Mo, Ta, Pt, Agand oxides of these as well as nitrides of these can be cited. Anexternal ohmic contact can be gained between the above and a p-typesemiconductor layer by annealing the above at a temperature of 400° C.,or higher. In particular, a multilayer film wherein Au is layered on Niis preferable. As for the total film thickness of the open electrode, 50Å to 10000 Å is preferable. In particular, in the case that a lighttransmitting electrode is used, 50 Å to 400 Å is preferable. Inaddition, in the case that a non-light transmitting electrode is used,1000 Å to 5000 Å is preferable.

Rhodium (Rh), iridium (Ir), silver (Ag), aluminum (Al) and the like canbe cited as a metal materials of a high reflectance which are used,specifically, in a reflecting electrode in a gallium nitride-based (atleast gallium and nitrogen are included) semiconductor light emittingdevice.

It is specifically preferable to use Rh as the material of the openelectrode. The electrode can be thermally stabilized and can have a lowlight absorption by using Rh. In addition, the contact resistance can belowered.

<3> Size and Form of Open Electrode

Though the relationships concerning the size of the openings of theelectrode and the size of the recesses or protruding portions on thesurface of the substrate are not specifically limited, it is preferablefor at least two or more edges of recesses or protruding portions to becreated within one opening. Thereby, the light scattered or diffractedby the unevenness can be effectively emitted and at the same time, theuniformity of the light emission increases.

In addition, the open electrode is an electrode having a plurality ofopenings that penetrate to the surface of the p-type semiconductor layerand that are surrounded by the electrode and it is preferable forL/S≧0.024 μm/μm² to be fulfilled wherein the area of a portionsurrounded by the outermost peripheral portion (total area of theelectrode including the openings) is S and the total sum of the lengthof the inner periphery of the openings is L. Thereby, a semiconductorlight emitting device can be gained wherein light can be efficientlyemitted from the surface of the p-type semiconductor layer to theoutside and, in addition, Vf is low.

It is preferable for the respective openings of the plurality ofopenings to have approximately the same form and thereby, the creationof openings becomes easy and the distribution of emitted light withinthe surface becomes uniform. In addition, it is preferable for therespective openings to have approximately the same area and thereby, thedistribution of the emitted light within the surface becomes uniform.

In the case that openings are formed in a thick layer, the shape, thesize and the like of these openings is controlled so that the efficiencyof the light emission can be enhanced and the efficiency of lightgeneration can be increased. In particular, the more efficient emissionof light becomes possible by controlling the length L of the innerperiphery of the openings. When L/S becomes small, that is to say, whenthe total sum L of the length of the inner periphery of the openingsbecomes small relative to the area S surrounded by the outermostperipheral portion of the open electrode, the output to the p-typesemiconductor layer side is lowered.

FIG. 13 shows the power conversion efficiency when the ratio of theopenings remains the same, that is to say, when the total area of theopenings remains the same while the length of the inner periphery ischanged. The area of the openings remains the same and thereby, thecontact area between the p-type semiconductor layer and the openelectrode remains the same so that Vf and the quantum efficiency areconsidered to be the same. It is understood from this figure that outputcan be enhanced by changing the length of the inner periphery of theopenings, even when the ratio of the openings remains the same. Then,according to the present invention, a semiconductor light emittingdevice of a high output can be gained by adjusting the length of theinner periphery of the openings in a range wherein L/S≧0.024 μm/μm² isfulfilled. Though the upper limit is not specifically set, in actualitywhen L/S becomes greater than 1 μm/μm², the size of one opening becomestoo small and the device becomes impractical.

The reason why the output efficiency from the p-type semiconductor layerside is greatly affected by the length of the inner periphery of theopenings rather than by the area of the openings as described above, isthat an intense emission of light is observed at the boundary betweenthe electrode and the p-type semiconductor layer and therefore, anenlargement of the boundary, that is to say, a lengthening of the innerperiphery of the openings allows the efficient emission of light. Inorder to further enlarge the boundary, the outermost peripheral portionof the p-side ohmic electrode is formed in a line of a non-linear naturealong the edge portion of the semiconductor layer, and thereby, thelength of the boundary of the p-side ohmic electrode and the p-typesemiconductor can be enlarged so that the output can be furtherincreased.

A plurality of openings as described above can be created so that therespective openings have approximately the same shape and thereby, theplurality of openings can be efficiently created. Furthermore, thedistribution of the openings within the surface are easily made uniformso that stable light emission can be gained. As for the shape ofopenings, a variety of shapes, such as rectangular, circular, triangularand the like can be used. The shape is preferably a square and aplurality of openings is created so that the openings are uniformlydispersed with constant spaces vis-à-vis the neighboring openings andthereby, it becomes easy to gain a uniform light emission. In addition,the plurality of openings is created so that the areas of the openingsbecome approximately the same and thereby, a preferred opening shape canbe selected depending on the position wherein an opening is created.

FIGS. 14A to 14D show preferred shapes of the open electrode. In FIG.14A, a p-side semiconductor layer 32 is formed on an n-sidesemiconductor layer 30 and an open electrode 34, which is a p-side ohmicelectrode, is formed on p-side semiconductor layer 32 and a p-side padelectrode 36 is formed as a portion of open electrode 34. In addition,an n-side pad electrode 38 is formed on n-side semiconductor layer 30that has been exposed through the etching of p-side semiconductor layer32. A plurality of circular openings is arranged in open electrode 34.FIG. 14B show open electrode with large size openings. FIG. 14C and FIG.14D only shows the opening electrode 34 and the pad electrode 36. Asshown in FIG. 14C, openings may be formed as slits, of which ends areopen. In this case, the ohmic electrode is like a combination of aplurality of line electrodes. The openings are preferably formed so thatcurrents are not concentrated locally. FIG. 14D shows a modified exampleof the shape of the openings, wherein a plurality of openings, in an arcform and arranged so as to be concentric, is provided with an n-side padelectrode (not shown) placed at the center. Such an opening shapeenhances the uniformity of the emitted light.

In addition, though the shape of the p-side ohmic electrode in the crosssection of the edge portion may be vertical, as shown in FIG. 15A, itmay, preferably, be a mesa (=trapezoid), as shown in FIG. 15B. In thecase, particularly, of a gallium nitride-based compound semiconductorlight emitting device, the device has a property wherein the intensityof the emitted light is high at the peripheral portion of the p-sideohmic electrode and therefore, such a cross sectional edge portion form,that is to say, a mesa (=trapezoid) allows light to be efficientlyemitted. In this case, it is preferable for the angle of taper θ of thecross sectional edge portion to be in the range of 30 degrees≦θ<90degrees. In the case that the angle of taper is 30 degrees or less, theresistance value of the p-side ohmic electrode becomes great in thetapered portion and therefore, it becomes difficult to effectivelyutilize the property that the peripheral portion of the electrode emitsintense light.

(2) Form of Semiconductor Light Emitting Device

According to the present invention, at least two semiconductor layersand a light emitting region, of which the materials differ from that ofthe substrate, are formed on the surface of the substrate in a layeredstructure. That is to say, the substrate and the semiconductor layersare made of different materials Here, in the case that an insulatingsubstrate is used as the substrate, for example, in the case that agallium nitride-based (at least gallium and nitride are included)semiconductor layer is formed on a sapphire substrate, an electrodecannot be formed on the substrate and therefore, it is necessary to formtwo electrodes of an n electrode and p electrode on the same side of thedevice. At this time, for example, a nitride semiconductor deviceformed, in this order, of an n-type semiconductor layer, a lightemitting region, a p-type semiconductor layer is formed. By etching aportion of the p-type semiconductor layer until the surface of then-type semiconductor layer is exposed. A p-side electrode is formed onthe surface of the p-type semiconductor layer and an n-side electrode isformed on the exposed surface of the n-type semiconductor layer so thatthe respective electrodes are placed at the two vertexes diagonallyopposite to each other of the semiconductor device in a square form, asshown in the top surface view of the semiconductor layer of FIG. 16.

In this case, light emitted to the outside from the sides of thesemiconductor light emitting device is blocked by external connectionterminals, such as the n-side electrode and a wire connected to then-side electrode, formed on the sides by exposing the n-typesemiconductor layer.

As shown in FIG. 17, n-type semiconductor layer exposed is locatedinside the p-type semiconductor layer so that the light emitting regionthat emits light between the n-type semiconductor layer and the p-typesemiconductor layer is provided on the entirety of outer sides of thesemiconductor light emitting device to increase the efficiency of lightemission to the outside of the device. In the case of a device wherein ap-type semiconductor layer, a light emitting region and an n-typesemiconductor layer are layered, in this order, on a substrate, theexposed surface of the p-type semiconductor layer is provided inside then-type semiconductor layer so that the same effects can be gained.

In addition, as shown in FIG. 17, in the case a inner portion ofone-type semiconductor layer is taken away with etching so thatanother-type of semiconductor layer is exposed, a branch electrodeprotruding from a pad electrode for diffusing current is preferablyformed on the exposed semiconductor layer. This uniformalize the currentflow in the one-type semiconductor layer. In the case that the electrodewith openings is formed, the branch of the pad may be formed on theelectrode. More preferably, the branch is formed along the outerperiphery of the semiconductor This further improves the uniformity ofthe light.

The external shape of the semiconductor light emitting device, as viewedfrom above, can be quadrangular, triangular or formed of other polygons.The exposed area of one-type semiconductor layer and the electrodeformed on the exposed layer is preferably formed so that a portionthereof extends toward the vertex of the light-emitting device. Thismakes current flow uniformly and such a configuration is preferablebecause light emission in the light emitting region becomes uniform.

In the case that a light emitting device of the present invention is,for example, a gallium nitride-based (at least gallium and nitride areincluded), mixture of fluorescent material including YAG and a resin arepreferably formed on the surface of the light emitting device, in orderto gain a white light emitting device having a high efficiency. A lightemitting device having a variety of wavelengths of emitted light andhaving a high efficiency of light emission is provided by appropriatelyselecting the fluorescent material.

The p-side electrode and the n-side electrode used in the presentinvention are the electrodes formed so as to contact with at least thesemiconductor layers and the materials thereof are appropriatelyselected to provide excellent ohmic properties for the contactedsemiconductor layers.

Example 1

A sapphire substrate, of which a C plane (0001) is used as the mainsurface, having the orientation flat in an A plane (11-20), is used asthe substrate.

First, an SiO₂ film 30 that becomes an etching mask is formed on asapphire substrate 10, as shown in FIG. 4A.

Next, a photomask of an equilateral triangle having a side of 5 μm isutilized and the photomask is arranged so that one side of theequilateral triangle becomes perpendicular to the orientation flat whilethe respective sides of the equilateral triangle become approximatelyparallel to (1-100), (01-10) and (−1010), that is to say, an M plane andthen, after SiO₂ film 30 and sapphire substrate 10 are etched by 3 μm to4 μm using RIE, as shown in FIGS. 4B and 4C, a repeating pattern ofprotruding portions 20 (hatched areas are unetched areas, that is tosay, protruding portions), as shown in FIG. 11B, is formed in thesurface portion of sapphire substrate 10 when SiO₂ film 30 is removed asshown in FIG. 4D. As for the length a of one side of a recess, a 5 μmand as for an interval b between a recess and a recess, b=2 μm. Thepitch between protruding portions (distance between the centers ofneighboring protruding portions) is 6.3 μm. In addition, the angle ofinclination of a side of a recess is 120 degrees.

Next, a buffer layer, which is made to grow at a low temperature, ofAL_(x)Ga_(1-x)N (0≦x≦1), of 100 Å, is layered as an n-type semiconductorlayer on sapphire substrate 10 wherein the repeating pattern ofprotruding portions 20 is formed and undoped GaN of 3 μm, Si doped GaNof 4 μm and undoped GaN of 3000 Å are layered and then, six well layersand seven barrier layers, having respective film thicknesses of 60 Å and250 Å, wherein well layers are undoped InGaN and barrier layers are Sidoped GaN, are alternately layered as an active layer of a multi quantumwell that becomes the light emitting region In this case, the barrierlayer that is finally layered may be of undoped GaN. Here, the firstlayer formed on the buffer layer grown at a low temperature is made ofundoped GaN and thereby, protruding portions 20 are uniformly filled inso that the crystallinity of the semiconductor layer formed on the firstlayer can have excellent properties.

After layering the active layer of a multi quantum well, Mg doped AlGaNof 200 Å, undoped GaN of 1000 Å and Mg doped GaN of 200 Å are layered asa p-type semiconductor layer. The undoped GaN layer formed as a p-typesemiconductor layer shows p-type characteristics due to diffusion of Mgfrom the neighboring layers.

Next, starting from the Mg doped GaN, the p-type semiconductor layer,the active layer and a portion of the n-type semiconductor layer areetched in order to form an n electrode so that the Si doped GaN layer isexposed.

Next, a light transmitting p-side electrode made of Ni/Au is formed onthe entirety of the surface of the p-type semiconductor layer and inaddition, a p pad electrode made of Au is formed at a position oppositeto the exposed surface of the n-type semiconductor layer and an nelectrode made of W/Al/W and an n pad electrode made of Pt/Au are formedon the exposed surface of the n-type semiconductor layer on the lighttransmitting p electrode.

Finally, the wafer is cut into chips of quadrangular form and mounted ona lead frame with reflectors to gain a 350 μm□ semiconductor lightemitting devices. This chip is mounted on a lead frame with a reflectingmirror to form a bullet-like LED.

The LED gained in such a manner have a light emission output to theoutside of 9.8 mW according to a lamp measurement for a forwarddirection current of 20 mA (wavelength=400 nm).

Comparison Example 1

As a comparison example, a light emitting device is formed in the samemanner as in first embodiment without the provision of unevenness on thesurface of the sapphire substrate and then, the light emission output tothe outside is 84 mW according to a lamp measurement for a forwarddirection current of 20 mA.

Example 2

A sapphire substrate, of which a C plane (0001) is used as the mainsurface, having the orientation flat in an A plane (11-20) is used asthe substrate.

A process in the substrate and layering of an n-type semiconductor layerto a p-type semiconductor layer are carried out in the same manner as infirst example.

Next, a p-type semiconductor layer made of Mg doped GaN, an active layerand a portion of the n-type semiconductor layer are etched in order toform an n electrode so that the n-type semiconductor layer made of Sidoped GaN is exposed.

Next, a photomask having a pattern wherein equilateral triangles havinga side of 5 μm, as shown in FIG. 16, are most densely filled per unitarea is utilized so that a light transmitting p electrode made of Ni/Auis formed on almost the entirety of the surface of the p-typesemiconductor layer.

Furthermore, a p-side pad electrode made of Au is formed at a positionopposite to the exposed surface of the n-type semiconductor layer on thelight transmitting p electrode and an n electrode made of Ti/Al and an npad electrode made of Pt/Au are formed on the exposed surface of then-type semiconductor layer.

Finally, the wafer is split into chips of quadrangular forms to gainsemiconductor light emitting devices. This chip is mounted on a leadframe with a reflecting mirror to form a bullet-Like LED.

The LED gained in such a manner has properties wherein the vicinity ofthe periphery of the p electrode emits light that is more intense thanthat from other portions and therefore, the light emission output isincreased in comparison with first embodiment.

Example 3

A sapphire substrate, of which a C plane (0001) is used as the mainsurface, having the orientation flat in an A plane (11-20) is used asthe substrate.

A process of the substrate and layering of an n-type semiconductor layerto a p-type semiconductor layer are carried out in the same manner as infirst example.

Next, starting from Mg doped GaN, the p-type semiconductor layer, anactive layer and a portion of the n-type semiconductor layer are etched,in order to form an n electrode so that the Si doped GaN layer isexposed.

Next, a photomask of a square pattern is utilized so as to form a pelectrode 104 made of Rh on almost the entirety of the surface of thep-type semiconductor layer. The shape of the openings is square, ofwhich side is 7.7 μm. The interval of the openings is 6.3 μm. Theaperture ratio of the opening is about 30%.

Furthermore, a p-side pad electrode made of Pt/Au is formed at aposition opposite to the exposed surface of the n-type semiconductorlayer on p electrode and an n electrode made of W/Al/W and an n padelectrode made of Pt/Au are formed on the exposed surface of the n-typesemiconductor layer.

Finally, the wafer is split into chips to gain semiconductor lightemitting devices. This chip is mounted on a lead frame with a reflectingmirror to form a bullet-like LED.

The LED gained in such a manner has properties wherein the vicinity ofthe periphery of the p electrode emits light that is more intense thanthat from other portions and in addition, a material having a highreflectance of the wavelength of the emitted light is used for theelectrode so as to reduce the light component absorbed by the electrode,and therefore, the light emission output is increased in comparison withfirst and second embodiments. The light emission output is 13.2 mWaccording to a lamp measurement.

Example 4

In the light emitting device of third example, p electrode is formed ina stripe form, as shown in FIG. 14C. By adopting such a stripe electrodestructure, a current supplied from a p-side pad electrode tosemiconductor layer is made uniform within the surface to increase theefficiency of the light emission.

The stripes of the first electrode are created as openings that exposesemiconductor layer and therefore, the length of the edge of theelectrode can be significantly increased and as a result, the efficiencyof the light emission is increased. At this time, it is preferable toachieve L/S≧0.024 μm/μm² wherein the value of S is gained by adding thetotal area Sa of openings 5 corresponding to the plurality of stripes,which exposes semiconductor layer, and area Sb of the electrode portionthat does not expose semiconductor layer, and the value of L is thetotal sum of the length of the circumferences of openings 5.

Example 5

A sapphire substrate, of which a C plane (0001) is used as the mainsurface, having the orientation flat in an A plane (11-20) is used asthe substrate.

A process of the substrate and layering of an n-type semiconductor layerto a p-type semiconductor layer are carried out in the same manner as infirst embodiment.

Next, the p-type semiconductor layer is etched until the Si doped GaNlayer is exposed from the inside of the p-type semiconductor layer, inparticular, from the center portion of the p-type semiconductor layer.The surface exposed as a result of etching at this time is formed sothat portions thereof are extended toward the three vertexes forming theshape of the semiconductor light emitting device, as shown in FIG. 17.

Next, a photomask of a pattern wherein equilateral triangles having oneside of 5 μm are most densely filled per unit area is utilized to form ap electrode 104 made of Rh in an equilateral triangular form on almostthe entirety of the surface of the p-type semiconductor layer.

Furthermore, a p pad electrode, which is also a p diffusion electrode,106 is formed of Pt/Au on p electrode 104. This p pad electrode, whichis also a p diffusion electrode 106, is provided by extending the padelectrode along the shape of the semiconductor light emitting devicethat becomes an equilateral triangle, as shown in FIG. 17. By providingthis electrode, it becomes easy for a current to uniformly flow throughthe entirety of the surface of the semiconductor layer and therefore,this electrode functions as a diffusion electrode.

In addition, an n electrode made of W/Al/W and an n pad electrode 103made of Pt/Au are formed on the exposed surface of the n-typesemiconductor layer.

Finally, the wafer is split into chips of equilateral triangular formsto gain semiconductor light emitting devices. Such a light emittingdevice is shown in FIG. 17, as viewed from above.

A light emitting device gained in such a manner has properties whereinthe vicinity of the periphery of the p electrode emits light that ismore intense than that from other portions and in addition, wherein amaterial having a high reflectance of the wavelength of the emittedlight is used for the electrode to reduce the light component absorbedby the electrode, and furthermore, wherein the light emitting region ofa multi quantum well structure is provided throughout the outer sides ofthe semiconductor light emitting device and therefore, the lightemission output is increased in comparison with first to thirdembodiments.

Example 6

A light transmitting resin containing Y₃Al₅O₁₂Y:Ce(YAG:Ce) having ayttrium aluminum oxide-based fluorescent substance as a base offluorescent material is formed on the top surface and on the sides of asemiconductor light emitting device gained in fifth example.

A semiconductor light emitting device gained in such a manner emitswhite light having a high light emission output.

Example 7

A sapphire substrate, of which a C plane (0001) is used as the mainsurface, having the orientation flat in an A plane (11-20), is used asthe substrate.

Next, following four types of protruding portions are made on thesurface of the substrate. The recess and protrusions are formed in thesame manner as Example 1.

(i) An equilateral-triangle like protrusions as shown in FIG. 11B areformed on the sapphire substrate. Each triangle is arranged so that oneside thereof is perpendicular to the orientation flat surface of thesapphire substrate. The triangles are arranged so that the vertexthereof heads inverse direction to the adjacent triangle. The length ofa side of the triangle is 5 μm and an interval between protrudingportions is 2 μm.(ii) A diamond like protrusions as shown in FIG. 11L is formed on thesurface of the substrate. The side length is 4 μm, and the intervalbetween protruding portions is 2 μm.(iii) A hexagon like protrusion as shown in FIG. 11M is formed on thesurface of the substrate. The side length is 3 μm, and the intervalbetween protruding portions is 2 μm.(iv) No protrusions are formed on the substrate surface.

Next, a buffer layer, which is made to grow at a low temperature, ofAL_(x)Ga_(1-x)N (0≦x≦1), of 100 Å, is layered as an n-type semiconductorlayer on sapphire substrate 10 wherein the repeating pattern ofprotruding portions 20 is formed and undoped GaN of 3 μm, Si doped GaNof 4 μm and undoped GaN of 3000 Å are layered and then, six well layersand seven barrier layers, having respective film thickness of 60 Å and250 Å, wherein well layers are undoped InGaN and barrier layers are Sidoped GaN, are alternately layered as an active layer of a multi quantumwell that becomes the light emitting region. In this case, the barrierlayer that is finally layered may be of undoped GaN.

After layering the active layer of a multi quantum well, Mg doped AlGaNof 200 Å, and Mg doped GaN of 200 Å are layered as a p-typesemiconductor layer.

Next, starting from the Mg doped GaN, the p-type semiconductor layer,the active layer and a portion of the n-type semiconductor layer areetched in order to form an n electrode so that the Si doped GaN layer isexposed.

Next, a light transmitting p electrode made of Ni/Au having thickness of60 Å/70 Å is formed on the entirety of the surface of the p-typesemiconductor layer and in addition, a p pad electrode made of Pt/Au isformed at a position opposite to the exposed surface of the n-typesemiconductor layer and an n electrode made of W/Al/W and an n padelectrode made of Pt/Au are formed on the exposed surface of the n-typesemiconductor layer on the light transmitting p electrode.

Light emitting power of bare chips in a wafer are measured with aprober. Results are shown in Table 1. In Table 1, the relative power isshown wherein the output-power of case (iv) is 1.

TABLE 1 relative power (i) triangle 1.48 (ii) diamond 1.43 (iii) hexagon1.48 (iv) flat 1

As shown in Table 1, more than 43% improvement is achieved with anuneven substrate. The light measurement without reflection mirrorsenhances the effect of the uneven substrate.

Finally, the wafer is cut into chips of 350 μm□ quadrangular form andmounted on a lead frame with reflectors to gain a bullet-like LED. TheVf and power (wavelength 460 nm) at 20 mA of the devices are as follows:

TABLE 2 V f (V) Power (mW) Relative power (i) triangle 3.54 10.08 1.14(ii) diamond 3.55 10.01 1.13 (iii) hexagon 3.51 10.30 1.16 (iv) flat3.48 8.85 1

As shown in Table 2, more than 13% improvement is achieved with anuneven substrate. The best result is achieved with hexagon protrudingportions.

Example 8

In this example, Rh electrode with openings is used alternatively. Otherconstructions except p-electrode are the same as those in Example 7. Theshape of the openings is square, of which side is 7.7 μm. The intervalof the openings is 6.3 μm. The aperture ratio of the opening is about30%.

The results with bare chips are shown in Table 3. In Table 3, therelative power is shown wherein the output-power of case (iv) is 1.

TABLE 3 relative power (i) triangle 1.54 (ii) diamond 1.56 (iii) hexagon1.65 (iv) flat 1

As shown in Table 3, more than 54% improvement is achieved with anuneven substrate.

The bullet-like LEDs, which emit 460 nm light, are formed to evaluatetheir Vf and output power at 20 mA. The results are shown in Table 4.

TABLE 4 V f (V) Power (mW) Relative power (i) triangle 3.87 12.74 1.17(ii) diamond 3.96 12.95 1.19 (iii) hexagon 4.08 13.06 1.20 (iv) flat3.97 10.85 1

As shown in Table 4, more than 17% improvement is achieved with anuneven substrate. Especially, the best results are obtained with thehexagonal protrusions.

As can be seen from Examples 7 and 8, a p-side electrode with openingscan cooperate with the unevenness of the substrate, and thereby theeffect of the unevenness is considerably improved.

Although the invention has been described in its preferred form with acertain degree of particularity, it is understood that the presentdisclosure of the preferred form has been changed in the details ofconstruction and the combination and arrangement of parts may beresorted to without departing from the spirit and the scope of theinvention as hereinafter claimed.

1. A semiconductor light emitting device, comprising: a substrate; a plurality of GaN based semiconductor layers stacked on said substrate and made of different materials from that of said substrate; and an ohmic electrode formed on the top layer of said GaN based semiconductor layers, so that light generated in said GaN-based semiconductor layers is emitted from said ohmic electrode or from said substrate, wherein a plurality of protruding portions for scattering or diffracting light generated in said GaN based semiconductor layers are created in the surface of said substrate in a repeated pattern with a pitch larger than or equal to λ/4 and smaller than or equal 10 μm, where λ is the wavelength of a light emitted from said GaN based semiconductor layers. a planar shape of said protruding portions is approximately triangular or hexagonal, each component side of the planar shape of said protruding portions is perpendicular to a line segment connecting a center and a vertex of a hexagon whose component sides are defined by A axes of said GaN based semiconductor layers, wherein a side surface of said protruding portions is inclined so that a taper angle of said side surface is greater than 90 degrees and smaller than or equal 150 degrees, and wherein the top and side surfaces of said protruding portions and flat portions, where no protruding portions are formed, on the surface of said substrate are continuously covered by said GaN-based semiconductor layers.
 2. A semiconductor light emitting device according to claim 1, wherein said ohmic electrode covers substantially the entire surface of the top layer of said GaN based semiconductor layers.
 3. A semiconductor light emitting device according to claim 1, wherein said ohmic electrode is transparent.
 4. A semiconductor light emitting device according to claim 1, wherein said substrate is selected from the group consisting of a sapphire substrate, an SiC substrate and a spinel substrate.
 5. A semiconductor light emitting device according to claim 1, wherein said substrate is a C-plane (0001) sapphire substrate.
 6. A semiconductor light emitting device according to claim 1, wherein a size of said protruding portions is greater than or equal to λ/(4n) and smaller than or equal to 20 μm, where λ is the wavelength of a light emitted from said GaN based semiconductor layers and n is the refractive index of said GaN based semiconductor layers.
 7. A semiconductor light emitting device according to claim 1, wherein the taper angle is greater than or equal to 105 degrees and smaller than or equal to 150 degrees. 